The present invention relates to polyurethanes showing improved physical properties. In particular, it relates to polyurethanes in which physical properties are enhanced by molecular reinforcement.
Polyurethanes are a highly versatile class of plastics which find use in a broad range of applications. The properties of various polyurethanes in many cases determines, and often limits, these applications. Thus, much research is directed toward improving the properties of different types of polyurethanes in order to better meet the needs of a specific end use. In particular, improvements in flexural strength/modulus, tensile strength, tear strength and moisture resistance are highly sought by those in the field.
One method of obtaining improvement in certain of the mechanical properties has been disclosed by Turner in U.S. Pat. No. 4,701,475. That patent discloses polyurethanes containing dispersed particles of high melting, rigid, rodlike polymer capable of increasing tensile strength and/or elongation and, in some instances, tensile modulus. Thomas et al., in U.S. Pat. No. 4,745,137, discloses a solution or dispersion of a polymer prepared from an ethylenically unsaturated polyaromatic compound which contains a rigid moiety comprising at least two aromatic nuclei which are connected by a rigid connecting group, in a compound having at least two isocyanate-reactive groups per molecule. U.S. Pat. No. 4,745,136 extends the scope of that invention to include solutions or dispersions of polymers prepared from ethylenically unsaturated steroid derivatives. Polyurethane slabstock foam prepared using the polymer solutions or dispersions exhibited improved indentation load deflection, modulus and tensile elongation. U.S. Pat. No. 4,745,135 discloses polyurethanes prepared from polyols containing one or more rodlike mesogenic moieties as a part of the backbone or as pendant groups. Exemplary are the hydroxyl-terminated aromatic polyesters and the mesogen initiated polyethers such as alkylene oxide derivatives of cellulose or a rigid 4,4'-bisphenol.
Tanaka et al. (Polymer Preprints, Japan, 33(7) 1647-50 (1984)) discloses reaction of di(p-oxymethylphenyl)terephthalate or of a bis(azomethine)diol, prepared via reaction of 1 mole of terephthaldehyde and 2 moles of p-aminophenethyl alcohol, with various diisocyanates to provide liquid crystalline polyurethanes. Tanaka and Nakaya (Kohunshi Ronbunshu, 43(5) 311-314 (May 1986)) disclose reaction of 4,4'-di(2-hydroxyethyloxy)biphenyl with various diisocyanates to provide liquid crystalline polyurethanes. Iimura et ai. (Makromol. Chem. 182, 2569-75 (1981)) and Japanese Patent Application No. 55-56968 disclose reaction of 3,3-dimethyl-4,4'-diisocyanatobiphenyl with various alkanediols to provide liquid crystalline polyurethanes. The reaction of 3,3'-dimethyl-4,4'-diisocyanatobiphenyl with di-, tri- or tetraethylene glycol was shown to produce non-mesomorphic (amorphous) polyurethanes.
One particular problem encountered in the polyurethanes discussed above, in which relatively substantial levels of rodlike mesogenic moleties are present in the main chain of the polyurethane, is that they possess relatively high melt (T.sub.m) and isotropization (T.sub.i) temperatures. These values exceed typical polyurethane processing temperatures and, in some cases, are even above the decomposition temperature of the urethane linkages. Thus, they represent impractical formulation variations.
Thus, it would be desirable in the art to prepare polyurethane compositions from formulations which are easily processed and which do not tend to decompose at processing temperatures. Furthermore, it would be desirable to prepare polyurethane compositions exhibiting one or more enhanced mechanical properties such as flexural strength/modulus, tensile strength or tear strength; or improved moisture resistance from such formulations.